The Universal Mobile Telecommunication System (UMTS) is one of the 3G mobile communication technologies designed to succeed GSM. 3GPP Long Term Evolution (LTE) is a project within the 3rd Generation Partnership Project (3GPP) to improve the UMTS standard to cope with future requirements in terms of improved services such as higher data rates, improved efficiency, and lowered costs. The Universal Terrestrial Radio Access Network (UTRAN) is the radio access network of a UMTS system and evolved UTRAN (E-UTRAN) is the radio access network of an LTE system. As illustrated in FIG. 1, a radio access network typically comprises user equipment (UE) 150 wirelessly connected to radio base stations (RBS) 110a-c, commonly referred to as NodeB (NB) in UTRAN and eNodeB (eNB) in E-UTRAN.
E-UTRA according to Release 8 (Rel-8) of the 3GPP specifications supports bandwidths up to 20 MHz. However, one of the requirements of future releases of this standard is expected to be the support of bandwidths larger than 20 MHz. A further important requirement on such releases is to assure backward compatibility with Rel-8. This should also include spectrum compatibility. That would imply that a future-release carrier, wider than 20 MHz, should appear as a number of Rel-8 carriers to a Rel-8 UE. Each such carrier is sometimes referred to as a Component Carrier (CC). In particular, for early deployments of future releases, it can be expected that there will be a smaller number of future-release UEs compared to many legacy Rel-8 UEs. Therefore, it is necessary to assure an efficient use of a wide carrier also for legacy UEs, i.e., that it is possible to implement carriers where legacy UEs can be scheduled in all parts of the wideband future-release carrier.
The straightforward way to obtain this would be by means of carrier aggregation. Carrier aggregation implies that a future-release UE can receive and send on multiple CCs, where the CCs have, or at least have the possibility of having, the same structure as a Rel-8 carrier. Carrier aggregation is illustrated in FIG. 2a, where five CCs 210, each of 20 MHz bandwidth, have been aggregated together to form an aggregated bandwidth 220 of 100 MHz. Carrier aggregation is planned for Release 10 (Rel-10) of the 3GPP LTE specifications.
Carriers can be aggregated contiguously, as illustrated in FIG. 2a, or they may be aggregated from discontinuous portions in the frequency domain (sometimes also called spectrum aggregation). FIG. 2b illustrates schematically an example with non-contiguous carriers.
With the carrier aggregation concept, it may be possible to support, among other things:                Higher bit-rates;        Farming of non-contiguous spectrum—i.e., to provide high bit-rates and better capacity also in cases when an operator lacks contiguous spectrum;        Fast and efficient load balancing between carriers.        
The LTE carrier or spectrum aggregation has some similarities with concepts such as Dual- or Multi-Carrier (DC or MC) HSPA, where one or multiple carriers in UTRAN are combined.
It should be noted that carrier aggregation can be viewed as a UE-centric concept, in that one UE may be configured to use, e.g., the two left-most CCs (230) in FIG. 2b, another UE may be configured to use only a single CC such as the right-most CC (250) in FIG. 2b, and a third UE may be configured to use all of the CC (230, 240, 250) depicted in FIG. 2b. Thus, a UE may be configured with component carriers (CCs), on a carrier of a specific frequency within the same frequency band or within different frequency bands. Multiple uplink (UL) and downlink (DL) CCs are configured independently of each other, meaning that they are not necessarily configured as UL/DL pairs as in Rel-8/9 of the 3GPP LTE specifications. Asymmetric configurations are possible, where the number of configured UL CCs differ from the number of configured DL CCs.
Initially, the UE will be configured with one UL/DL CC pair on which it makes the initial random access. These CCs are called Primary Component Carriers (PCC). In addition to the UL/DL PCC pair, the eNB may configure the UE with additional CCs, so called Secondary Component Carriers (SCC) as needed.
Conventionally, a carrier is a portion of the frequency spectrum that can be used for transmission in UL and/or DL. The notion of cell is normally used to denote the radio network object that can be uniquely identified by a UE. In UTRAN, e.g., a cell is identified thanks to a cell identification that is broadcasted over a geographical area from one UTRAN access point. Typically, a cell is associated with a single pair of UL and DL carriers in FDD, and a single carrier that provides both UL and DL resources, if the mode is TDD. There may be multiple cells associated with one Carrier, as long as the cells are physically separated from each other. This is the case, e.g., when neighbouring eNBs each implement cells on the same carrier.
As already mentioned above, future releases of e-UTRAN introduces the support of a larger bandwidth or frequency spectrum, and for compatibility reasons the extra bandwidth may be seen as additional carriers of, e.g., 20 MHz, so-called component carriers (CC) that are aggregated together. However, by applying the notion of cell being the radio network object associated with a certain CC—a cell that may be identified by the UE—a UE using carrier aggregation in connected mode may also be referred to as being connected to multiple aggregated cells: one primary cell (referred to as PCC above, and for which notations such as PCell, primary serving cell and serving cell are also used), and additional configured CCs that are part of another set of secondary cells (referred to as SCC above, and for which notations such as SCell, secondary serving cell, and secondary cell are also used).
In LTE, many different scenarios and carrier types are being discussed, including the aggregation of Rel-8 backwards compatible carriers. Also, non-backwards compatible and extension carriers are being discussed. Such carriers may not be available for Rel-8 terminals. A particular and relevant example of a plausible carrier aggregation scenario includes the case when two or more Rel-8 downlink carriers/cells, are aggregated for a UE. It should be noted that carrier aggregation is typically and mainly relevant for a connected UE, which is a UE that is actively involved in transmission to and from the eNB, and thus has a connection with the eNB controlling the aggregated carriers/cells.
The aggregated carriers/cells may thus also be available for Rel-8 UEs, meaning that each of the carriers/cells may be independently available for single-cell operation. Such a single-cell operation includes idle mode camping when the UE is typically inactive, and connected mode operation in single-cell mode. Therefore, these Rel-8 compatible carriers/cells will have to provide System Information (SI) that is broadcasted in the cell, such that UEs may perform, e.g., idle mode camping and cell selection according to the rules set by the parameters broadcasted in SI. Other sets of particular relevance that also must be broadcast in each of the Rel-8 carriers/cells are parameters related but not limited to:                Random Access (RA), and RA channel (RACH) parameters, i.e., common parameters that define how a UE should access a cell;        UL parameters, i.e., common parameters e.g., related to UL bandwidth, frequency, PUCCH (Physical Uplink Control Channel), and PUSCH (Physical Uplink Shared Channel);        DL parameters, i.e., common parameters e.g., related to PFICH (Physical Control Format Indicator Channel), PDSCH (Physical Downlink Shared Channel), paging information, and DL frequency and bandwidth.        Cell-specific timers and constants.        
SI also includes, e.g., sets of parameters related to cell and radio access technology (RAT) selection. “Common parameters” is used to denote parameters that many or all UEs in a cell are required to acquire according to specific rules in specifications. Such common parameters will typically be read and used by many UEs. Deviations from this general rule may be specified.
In Rel-8, the SI of relevance for a connected mode UE is distributed in the Master Information Block (MIB), and the first two System Information Blocks (SIB1 and SIB2). It may be specified that the UE should maintain updated information of this required SI, as specified in the 3GPP standard. When the SI changes the UEs are notified by different means, in order for them to re-acquire the required Si.
It is possible that also non-backwards compatible carriers/cells may be available for idle mode camping and single-cell operation. In this case these carriers/cells will also have to broadcast SI with sets of cell-specific parameters similar to the ones described above.
The DL of a cell will typically include broadcast of SI parameters that are relevant for this cell, including parameters of relevance both for the DL and an UL. Technically, it would be possible to, in addition to the aforementioned parameters, broadcast parameters associated with a second cell on the DL of a first cell. However, such a solution may not be preferable, since the parameters associated with the second cell should then often also be broadcast on the DL of the second cell. This duplication is not desirable, and 3GPP has therefore agreed to, in Rel-10, not broadcast information related to a second cell on a first cell.
In a typical use-case, illustrated by the signaling diagram of FIG. 3a, a UE 310 will first be connected to a single cell, also called the primary cell, following an RRC connection setup procedure 301 similar to those known from Rel-8. Only then may the eNB 320, based on different criteria, decide to configure the UE 310 for reception (DL) and transmission (UL) on multiple aggregated carriers/cells. This means that the eNB 320 may send a configuration message, typically an RRCConnectionReconfiguration message 302, including information about the additional UL and DL CCs that the UE is supposed to take into use. The UE replies to the configuration message, typically with an RRCConnection ReconfigurationComplete message 303.
Such a connected mode UE will now have knowledge of multiple UL and DL CCs, which may aggregate up to a very large bandwidth, and the UE is now ready to be scheduled on all of the CCs—sometimes on individual CC, and sometimes on all CC at the same time. There is now thus SI available on multiple DL Component Carriers or cells that the UE has been configured with. However, as described above, this required SI is of relevance also for UEs operating in single-cell mode, such as for Rel-8 UEs that lack the capability for carrier aggregation. It may happen that some of the SI that is of relevance for such single-cell operation is not valid, not useful, and possibly even harmful as it may result in unwanted restrictions to the flexibility of carrier aggregation operation, as will be further described below. The problem is thus that SI of relevance for single-cell operation is broadcasted in all cells in order to, e.g., provide backwards compatibility, but when a cell should be used for carrier aggregation this single-cell SI is not optimal.
Assume now the example above, where a UE has first been connected to one cell—the primary cell—that includes both a configuration of a DL and an UL, and that the UE is configured to aggregate one additional DL carrier of a secondary cell. Assume further that SI of relevance to the DL carriers is broadcasted in both aggregated cells. However, as noted above, the SI broadcasted on each of the cells will provide SI of relevance also for single-carrier operation. Thus, the SI broadcasted in the secondary cell will include information about a corresponding UL configuration, including e.g., UL bandwidth and frequency, RACH, PUCCH and PUSCH parameters. Thus, if the UE operating with multiple aggregated carriers is required to read and follow the SI broadcasted on both the primary and the secondary cell, it will result in a situation where the UE by necessity also configures all the corresponding parameters for single-cell operation.
In the following example, an RBS is in control of three UL and DL carriers in three cells, here denoted UL1/DL1/cell1, UL2/DL2/cell2, and UL3/DL3/cell3. When operated in single-cell mode, the carriers are coupled such that UL1 is operated together with DL1, UL2 with DL2, and UL3 with DL3, respectively. Thus, any SI of relevance for ULx/DLx is broadcast on cellx, as illustrated in FIG. 2c. Note that the example does not rule out that ULx is on the same frequency as DLx, which is the case for a TDD mode.
It is assumed that a UE is connected to the cell2 defined by DL2, i.e., the UE 310 follows known Rel-8 procedures, reads required SI on DL2, and uses UL2 for UL transmissions (state 304 in signaling diagram of FIG. 3b). Furthermore, the eNB 320 now wants to configure the UE 310 with two additional DL carriers, since the eNB concludes that the UE needs a larger DL bandwidth. The eNB sends a configuration message in 305 including information that the UE may additionally use DL1 in cell1 and DL3 in cell3, in addition to the already existing DL2. However, if the UE now reads the required SI broadcasted in cell1 and cell3 as well, following existing art, the UE will take also the corresponding UL configurations into use, i.e., UL1 and UL3. This was clearly not desirable, as the eNB only found reasons to aggregate DL carriers in this case, and not to configure the UE with additional UL bandwidth that greatly exceeds the needs of the UE. Thus, existing art provides inflexibility, in that SI of relevance for single-cell operation may be too restrictive for carrier aggregation operation.
Another example concerns, e.g., the Random Access (RA) configuration. Assume now that the UE has been configured with two aggregated cells in both UL and DL, say UL1/DL1 in cell and, UL2/DL2 in cell2. The SI parameters concerning carrier frequency and bandwidth broadcasted on both the cells are in this case of relevance. However, both cells' SI includes RA parameters, offering the UE a possibility to perform RA on both of the UL carriers. It may be that the eNB wants to constrain the UE to perform RA only on one particular of the available ULs. With present art, this is not possible, since the UE will read SI comprising RA parameters on both cells and thus perform RA on both ULs.
A further example concerns, e.g., PUCCH control. It has recently been agreed that it shall be possible to provide all PUCCH control information on one single UL carrier, regardless of how many UL and DL carriers that are configured for a UE. In the examples illustrated above, each cell will provide independent PUCCH parameters. However, the UE should only follow PUCCH parameters broadcasted on one of the cells.
Yet another example concerns timers and constants. Each of the aggregated cells may provide independent timer and constant values that might be different in value. However, the timers and constants may not be relevant per cell, but rather per UE. A UE may e.g., maintain only a single timer t1 that expires when the value T1 is reached, but SI on all the cells is offering different values for this T1, and it is unclear which one of the values that the UE should apply.